Which of the following statements about topical anesthetics is true?

Topical anesthetics are indeed used primarily to provide localized pain relief on surface tissues. While they can offer quick relief, they generally do not provide immediate effects compared to the more rapid onset associated with needle injections.

In terms of the effectiveness on keratinized tissues, topical anesthetics tend to have limited efficacy in these areas due to the barrier properties of keratinized skin, which can impede penetration. Thus, although they might provide some level of anesthesia, it's often less effective than in non-keratinized tissues.

When applied to mucosal surfaces, topical anesthetics are widely utilized, especially in dental settings. Therefore, suggesting that they should not be used for mucosal applications is misleading, as they are commonly used in those instances to alleviate discomfort.

Considering all of this, while topical anesthetics offer benefits, they typically require more time to take effect than the immediate response that needle injections provide. This is owing to the need for absorption into the tissue, making the statement about their onset time the most accurate among the options provided.